Concern over the possible depletion of fossil fuel energy sources has generated interest in recent years in the search for and development of alternative energy sources. Numerous alternative energy sources have been contemplated including solar energy utilized as electricity either directly through photovoltaic devices or indirectly through thermal devices. The former has not received as much attention as the latter which will, as presently contemplated, use semiconductor devices. Such devices are presently too expensive, compared to other sources of electricity, for commercially successful widespread utilization.
Considerable effort has, therefore, been expended in attempts to reduce the cost of photovoltaic devices. For example, much effort has been directed toward reducing the cost of silicon solar cells having p-n junctions. This effort is attractive because silicon is both abundant and cheap. Other efforts have been directed at other materials and other types of cells. For example, solar cells in which the active part of the cell is a junction formed at a liquid-solid interface have been examined. These cells are commonly referred to as semiconductor liquid junction cells. The characteristics of this type of cell are discussed by Gerischer in The Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, Vol. 58, pp. 263-274 (1975).
Although many materials have been investigated for use in such cells, the highest efficiency, approximately 12 percent, has been obtained in cells using a ruthenium treated n-type GaAs photoactive electrode and a selenide-polyselenide redox couple. These cells are described in U.S. Pat. No. 4,182,796 issued to Adam Heller, Barry Miller and Bruce A. Parkinson on Jan. 8, 1980.
The use of p-type electrodes in such cells is interesting because illumination tends to protect the semiconductor from surface oxidation which is an obstacle to stability in n-type cells. In cells using p-type electrodes, electrons drift to the electrode-electrolyte interface and reduce an oxidized species in the electrolyte. Many p-type semiconductors, including GaP, GaAs, Si, Ge, CdTe, and InP, have been investigated in both aqueous and nonaqueous electrolytes. None of these cells has approached the efficiency achieved with the above described n-type GaAs system although substantial photovoltages have been noted for strongly reducing solutions such as Eu(III)/EU(II) in acid. In some cases, the cell voltages have been observed to be insensitive to the redox potentials and it was thus concluded that the cells would be of limited value.